Xorg memory leak?

for about a week or so I'm seeing the xserver consuming rather large
amounts of memory on my F11 system. Right now top shows it using 1.4g
resident memory (1.8g virtual). Usually the pixmap use of Firefox is the
main culprit in such cases but even after quitting I end up with the above
memory usage. Xrestop shows a usage total of merely 30MB.
Is there a way to find out what this memory is used for? Right now I have
to log out and back in again in order to get my memory back. Could this be
a driver issue? I'm using the radeon driver with a 4850 card (r700).